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Full text of "An investigation of substrate effects on type two hot corrosion of marine gas turbine materials."

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Dudley Knox Library, NPS 
Monterey, CA 93943 



NAVAL POSTGRADUATE SCHOOL 

Monterey, California 








THESIS 






AN 
ON 


INVESTIGATION OF SUBSTRATE 
TYPE TWO HOT CORROSION OF I 
GAS TURBINE MATERIALS 

by 

Michael J. Shimko 
June 1983 


EFFECTS 
MARINE 


Th< 


3sis i 


Advisor: D.E 


. Peacock 



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An Investigation of Substrate Effects 
on Type Two Hot Corrosion of Marine 
Gas Turbine Materials 



3. TYPE OF REPORT 4 PE-RIOO COVERED 

Master's Thesis 
June 1983 



6. PERFORMING ORG. REPORT NUMBER 



7. AUTHOR^ •; 

Michael J. Shimko 



8. CONTRACT OR GRANT NUMBERS.) 



t. PERFORMING ORGANIZATION NAME ANO AOORESS 

Naval Postgraduate School 
Monterey, California 9394-0 



10. PROGRAM ELEMENT. PROJECT, TASK 
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June 1983 



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IS. SUPPLEMENTARY NOTES 



It. KEY WOROS (Contlnua on rawataa alda> II naeaaaarr and Idantlty by block numbar) 

Marine Gas Turbines 
Hot Corrosion 
Substrate Effects 
Turbine Blade Coatings 
CoCrAlY 



20. ABSTRACT (Contlnua on ravaraa alda II nacaaaawy and Idantlty by block numbar) 

CoCrAlY coated Modifications of Rene 80 (a Ni base superalloy) 
were tested for resistance to Type Two (Low Temperature) Hot 
Corrosion. The effects of Ti and Hf in the substrate (normally 
0.0,0 and 0.0,* respectively) and the presence of a Pt underlaver 
were investigated. 

Certain trends were distinguishable from the data obtained. 
Titanium alone was found to be beneficial, Titanium in 



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20. ABSTRACT (cont'd) 

conjunction with a platinum underlayer was found to be 
detrimental while platinum underlayers in conjunction with 
low titanium concentrations in the substrate were found to be 
beneficial. Hafnium had a noticeable, but irregular effect 
only on specimens with intermediate titanium concentrations. 
All the above effects were found to be diffusion related. 

This study also made certain refinements to the NPS Hot 
Corrosion Test Program and direct correlation of data obtained 
from different runs is now justified. 



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An Investigation of Substrate Effects on 
Type Two Hot Corrosion of Marine Gas Turbine Materials 



by 



Michael J. Shimko 
Lieutenant, United States Navy 
B.S., University of Maryland, 1977 



Submitted in partial fulfillment of the 
requirements for the degree of 



MASTER OF SCIENCE IN MECHANICAL ENGINEERING 



from the 

NAVAL POSTGRADUATE SCHOOL 
June 1983 



ABSTRACT 

CoCrAlY coated modifications of Rene 80 (a Ni base - 
superalloy) were tested for resistance to Type Two (Low 
Temperature) Hot Corrosion. The effects of Ti and Hf in 
the substrate (normally 5.0$ and 0.0$ respectively) and 
the presence of a Pt underlayer were investigated. 

Certain trends were distinguishable from the data 
obtained. Titanium alone was found to be beneficial, 
titanium in conjunction with a platinum underlayer was found 
to be detrimental while platinum underlayers in conjunction 
with low titanium concentrations in the substrate were found 
to be beneficial. Hafnium had a noticeable, but irregular 
effect only on specimens with intermediate titanium concen- 
trations. All the above effects were found to be diffusion 
related. 

This study also made certain refinements to the NPS 
Hot Corrosion Test Program and direct correlation of data 
obtained from different runs is now justified. 



TABLE OF CONTENTS 

I. INTRODUCTION - . 10 

A. HISTORICAL 10 

1. Naval Experience with the Gas Turbine ... 10 

2. Superalloys 12 

3. Coatings 13 

B. HOT CORROSION 16 

1. Hot Corrosion Testing 18 

2. Previous Research 21 

C. OBJECTIVES 22 

II. PROCEDURE 23 

III. DISCUSSION/RESULTS 26 

A. SUBSTRATE EFFECTS 28 

IV. CONCLUSIONS AND RECOMMENDATIONS 36 

APPENDIX A: TABLES 38 

APPENDIX B: FIGURES U 

LIST OF REFERENCES 64 

INITIAL DISTRIBUTION LIST 66 



LIST OF TABLES 

I. Nominal Chemical Composition of Rene' 80 and 

BC-21 38 

II. Test Parameters 39 

III. Listing and Results of Duplicate Pins 4-0 

IV. Listing of Samples Tested - Substrate Study .... O 

V. Corrosion Results - Substrate Study 4-2 



LIST OF FIGURES 

B.1 Relative Temperature and Pressure Profile of a 

Marine Gas Turbine Engine 4.3 

B.2 Simplified Drawing of the Electron Beam Physical 

Vapor Deposition (SB-PVD) Process 4.4. 

B.3 Typical CoCrAlY (BC-21) Coating on Rene' 80 . ... 45 
B.4- Type 2 (Low Temperature) Hot Corrosion Simplified 

Schematic 4.6 

B.5 Typical Type 2 Hot Corrosion in CoCrAlY (BC-21) 

Coating 4.7 

B.6 Typical Type 2 Hot Corrosion on BC-21 Coating - 

Macrophoto (enlarged 7.5 x) 4.8 

B.7 Cross Section of a Tube Furnace 4.9 

B.8 Schematic Illustration of the Substrate/Coating 

Diffusion Process 50 

B.9 Schematic Illustration of the Method of Using High 

Magnification Spectrochemical Analysis for Diffusion 

Study 51 

B.10 The Effect of S0 2 Flow Rate on Type 2 Hot Corrosion 

of BC-21 Coated Rene' 80 52 

B.11 Type 2 Hot Corrosion Behavior of BC-21 Coated Rene 1 

80 Modifications, Effect of Titanium 5A 

B.12 Type 2 Hot Corrosion Behavior of BC-21 Coated 

Rene' 80 Modifications, Effect of Platinum 57 

B.1 3 Type 2 Hot Corrosion Behavior of BC-21 Coated Rene' 

80 Modifications, Effect of Hafnium 59 



3.14- Chemical Spectrums of Center of BC-21 Coated and 

Corroded Rene' 80 (5% Ti Modification) 62 

B.15 Chemical Spectrums of BC-21 Coating on Corroded 
Rene' 80 (5% Ti Modification) with Platinum 
Underlayer 63 



8 



ACKNOWLEDGMENT 

The author would like to thank the staff of the 
Mechanical Engineering Department for their timely and 
efficient assistance and in particular to Adjunt Professor 
David E. Peacock, Adjunct Professor David H. Boone, and 
Mr. Tom Kellogg whose enthusiasm, help, and encouragement 
were instrumental in the completion of this thesis. 

A special thanks to my loving wife, Donna, and our 
recently arrived Jackelyn for their inspiration, support, 
and sacrifice throughout my stay at the Naval Postgraduate 
School. 



Du* 

Mo: 



I. INTRODUCTION 

A. HISTORICAL 

1 . Naval Experience with the Gas Turbine 

The United States Navy is currently pursuing one of 
the most ambitious shipbuilding programs since the end of 
World War II. In the last 10 years all new combatants have 
relied on either nuclear power or the gas turbine as their 
source of propulsive power. 

The Engine chosen for development and use by the 
Navy was the CF6/TF39 aircraft engine used on the C5 Transport 
aircraft. The marinized version of this engine has been 
designated the LM2500. The LM2500 is currently used in or 
scheduled for use in 30 DD-963 Spruance Class Destroyers, 
k DDG-993 Kidd Class Guided Missle Destroyers, 50 FFG-7 
Perry Class Guided Missile Frigates, the CG47 Tichondoroga 
Class Guided Missle Cruisers, the DDG-51 Class Guided 
Missle Destroyer, numerous hydrofoils and Surface Effect 
Ships, and a large number of commercial industrial and marine 
applications as well. 

Over the years gas turbine efficiency has grown 
through increased technology and design achievements but has 
been always limited by the high temperature materials used 
within the gas turbine, specifically within the high pressure 
turbine area which immediately follows the combustor assembly. 



10 



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m 



A simplified schematic and relative temperature and pressure 
profile of the LM2500 is shown in Figure B.1. 

The United States first use of the gas turbine was 
on the GTS CALLAGAN in 1967 when extensive testing of 
several versions of gas turbines was undertaken. In 1973 
the Navy committed itself to the LM2500 engine. The first 
tests of the LM2500 involved primarily long term high power 
runs, and in 1971 the initial DD power cycle testing was 
begun. These tests were thought to provide the most arduous 
operating environment for initial evaluation. The lifetime 
of the critical turbine blades was found to be approximately 
7000 hours. The limiting factor was Hot Corrosion of the 
blade coating which was known to occur at temperatures over 
850°C. 

In 19 7 3, partly in response to the oil embargo, a 
test cycle which reduced the average speed to 1 9 knots was 
started as a fuel conservation measure. Since at low power 
the maximum temperature of the gas turbine is less than for 
high power, it was predicted that the lifetimes of the 
critical gas turbine blading would be extended in this 
operating environment (this follows from classical arrhenius 
kinetics considerations). Unexpectedly, the blading lifetime 
was significantly reduced to less than 5000 hours. This was 
the Navy's first experience with Low Temperature Hot Corrosion 
(often referred to as Type 2 Hot Corrosion) {Ref. 1}. 
Subsequent changes in the operating cycle of CALLAGAN further 



11 



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reduced the time spent at full power from 60 to 18$, and the 
resulting turbine blade lifetimes diminished even further. 
Since these initial tests, blade lifetimes have been increased 
by improvements in intake air filtration systems to better 
remove sea salt spray. However Type 2 Hot Corrosion is 
still limiting blade life to 5000 hours, compared to 7000 
hours if Type 2 Hot Corrosion were not a factor. 

Given the need for significant amounts of time spent 
at low power operations while maintaining the capability to 
run at full power as mission needs dictate, Type 2 Hot 
Corrosion will continue to be a factor in the future. 
2. Superalloys 

In the development of the gas turbine, material 
selection of the critical high temperature and pressure 
components has been based primarily on mechanical behavior 
criteria (creep resistance, high temperature strength, etc.), 
and use of protective coatings to provide additional resis- 
tance to the corrosive environment. 

The superalloys are a class of iron, nickel and 
cobalt based alloys with various other elements added to 
achieve high temperature creep resistance, high temperature 
tensile strength, resistance to mechanical and thermal 
fatigue, as well as resistance to oxidation and hot corrosion 
{Ref. 2}. 

In nickel based superalloys, desired high temperature 
properties are obtained by the formation of a coherent 
second phase, gamma prime {Ni~(Al,Ti)} in a continuous 

12 



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nickel matrix, gamma {Ref. 3} . Although both phases have 
fee structures, slightly different lattice parameters result 
in coherency strains which results in an increment of 
strengthening. In general, the more gamma prime phase 
present, while still maintaining a continuous gamma phase, 
the better the mechanical properties. 

An increase in the amount of gamma prime can be 
achieved by a reduction in chromium content and an increase 
in the amount of titanium and/or aluminum {Ref. U }. Since 
chromium also enhances grain boundery strengthening, this 
results in one of many tradeoffs. 

The effect of high chromium then is a lower strength 
at high temperatures compared with alloys with a lower chromium 
content but with other solid-solution strengthening elements 
such as tungsten and molybdenum. Chromium and aluminum 
both form protective oxides which result in improved oxida- 
tion and hot corrosion resistance {Ref. 5} . 
3. oatings 

The alloy additions which confer the desired high 
temperature strength of superalloys generally lower their 
resistance to hot corrosion, oxidation, and thermal fatigue 
{Ref. 6}. Surface coatings are used to improve environmental 
resistance. This is usually accomplished by the formation 
of a protective oxide such as AlpOo and/or Cr^O^. 

The primary basis for selecting a protective coating 
is its inherent environmental resistance (i.e., its ability 



13 



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to form the required protective oxide). However, since it 
has been demonstrated {Ref. 7} that the coating and substrate 
can influence each other, the selection process requires that 
the coating and substrate be considered together as an 
integral system. However, the possible removal of the 
coating by wear or FOD (foreign object damage) and the 
difficulty of coating some interior surfaces of gas turbine 
airfoils requires that the uncoated basemetal should provide 
a minimal degree of corrosion resistance. A first consider- 
ation in this respect is similar or at least compatible 
mechanical properties. For this reason current coatings 
in use today are either Aluminide Diffusional Coatings or 
Metallic Overlay Coatings. 

Aluminide diffusional coatings are formed by diffusion 
aluminum into the surface of the substrate and converting 
them into an intermetallic compound. The resulting coating 
consists of an inner reaction-diffusion zone and one or two 
outer zones of intermatallic compounds of the metal aluminide 
type {Ref. 6}. Upon oxidation exposure, an aluminum oxide 
film forms on the surface and is the primary barrier against 
further oxidation. This oxide is reformed as required by 
the underlaying aluminide. 

Aluminide coatings are brittle at low and intermediate 
temperatures, and provide only moderate Hot Corrosion 
resistance when compared with most overly coatings. Duplex 
coatings (Modified Aluminide Coatings) have been developed 



U 



Du- 
Mo 



which have shown enhanced corrosion resistance. This has been 
accomplished by the addition of elements such as chromium, or 
noble metals such as platinum to the aluminide coating." 

The limitations of the aluminide coatings: brittleness, 
moderate corrosion resistance, and a strong substrate dependence 
have led to the development of the Metallic Overlay Coatings. 
These coatings are often of the MCrAlY type (where M = Fe, Ni, 
and/or Co) and are primarily applied by the Physical Vapor 
Deposition (PVD) process. A simplified schematic of one 
form of PVD process, the electron beam PVD (the process used 
for the coating of samples for this study) is shown in Figure 3.2. 
These coatings consist of two phases; a brittle aluminide phase 
in a ductile, chromium rich solid-solution matrix. A typical 
overlay coating, BC-21 , is shown in Figure 3.3. This class 
of coatings contain from U to 13% Al , 18 to 1^0% Cr, and 0.1 
to 0.5$ I with the balance either Co and/or Ni. The aluminum 
and chromium are protective oxide formers and the yttrium 
enhances oxide adhesion. The ability to vary the composition 
of these coatings for specific applications is a significant 
advantage over aluminide coatings. The composition of BC-21 
is given in Table I (along with the composition of Rene' 80, 
a Ni based superalloy) . BC-21 is the coating on the first 
and second stage turbine blades of the LM2500. It has a 
relatively high Cr content, 20 to 24-$, which enhances Hot 
Corrosion resistance {Ref. 8}. This necessitates a lower Al 
content to maintain sufficient ductility. The beneficial 



15 



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effects of platinum in aluminide coatings has led to testing 
of Overlay Coatings containing platinum and also of Overlay 
coatings applied over platinum underlayers. 

A third type of coating currently being studied is 
the Ceramic Coating. This offers the dual advantage of good 
corrosion resistance and high thermal resistance. This could 
allow increased turbine inlet temperatures and/or reduce 
cooling air requirements. Ceramic coatings have not yet 
been developed with sufficiently compatible mechanical 
properties for full airfoils and are not yet in commercial 
use {Ref. 9}. 

B. HOT CORROSION 

The surface degradation of marine gas turbine materials 
can be the result of several corrosion mechanisms. These may 
act singly, independently, or in combination. The known 
mechanisms are: oxidation, catostropic oxidation, high 
temperature hot corrosion, and low temperature hot corrosion. 
Specific morphologies have been identified that occur by 
some of the mechanisms above. Type 1 morphology is charac- 
teristic of the attack under conditions of high temperature 
hot corrosion on CoCrAlY type coatings. Type 2 morphology 
occurs in CoCrAlY type coatings under low temperature hot 
corrosion conditions. Type 3 morphology seems to occur under 
a combination of high temperature and low temperature hot 
corrosion conditions and/or in environments with a high SO., 
partial pressure. Based on these morphologies, the two main 



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forms of hot corrosion are now more correctly termed Type 1 
(high temperature) Hot Corrosion, and Type 2 (low temperature) 
Hot Corrosion. Their shorthand abbreviations, HTHC and LTHC 
are commonly used and will be used throughout this study. 

Type 1 Hot Corrosion has been known since the mid- 
1950's. It is associated with gas turbines used in jet 
aircraft which are usually run at high power levels. HTHC 
occurs in a temperature range above about 850 C. It requires 
a molten salt (Na ? S0.) film and a specific range of partial 
pressures of Op and S0~ and results in the dissolution of the 
protective oxide and the formation of a characteristic zone 
of aluminum depletion in advance of the corrosion front 
{Ref. 10}. Since Na-SO, has a melting point of 886°C 
Hot Corrosion was not expected to be a problem at temperatures 
much below this. In 197$, observations on the GTS CALLAGAN 
showed otherwise {Ref. 1}. 

Type 2 Hot Corrosion attacks CoCrAly coatings without 

preference to phase. It also requires a molten salt, but 

in this case it is an eutectic mixture (Na o S0, and MSO , ) 

<c 4 4 

which can have melting points as low as 575 C. LTHC also 

requires gaseous SCU , the partial pressure of which has been 

shown to be critical to LTHC attack {Ref. 11}. In more 

detail, Cobalt oxides formed on the coating react with 

gaseous SO ^ to form CoSO, which is absorbed by the Na ? S0 , 

in the molten salt mixture. As the CoSO, dissolves, the 

4 

melting point of the mixture is further reduced, until 



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at 50$ CoSO. an eutectic point at 560 C is reached {Ref. 12}. 
As the alloy begins to react with the molten salts, oxygen 
is removed from the molten salt phase and partial pressure 
gradients of 0~ and SO.-, are developed across the liquid, 
and SO., thus supplies the oxygen to react with elements 
in the alloy. 

Aluminum and sulfite ions react in areas of low oxygen 
partial pressure and aluminum is selectively removed from 
the coating and precipitated as aluminum oxide in areas of 
high oxygen partial pressure. This process is classic 
acidic fluxing and results in the severe pitting attack 
associated with LTHC. Figure B.h shows a simplified 
schematic of the mechanism described above, Figure B.5 shows 
a photomicrograph of a typical example of the pitting 
attack, and Figure B.6 shows a macrophoto of typical Type 2 
Hot Corrosion. Despite the lower temperature at which it 
occurs (compared to Type 1 attack), LTHC is generally more 
severe. This is partially due to the good wetting ability 
of the molten eutectic salt mixture which enables LTHC to 
attack the coating at microscopic imperfections in the protec> 
tive oxide layer. 

1 . Hot Corrosion Testing 

Hot Corrosion Testing in the laboratory involves 
the use of accelerated tests in order to duplicate the 
corrosion of possibly 5000 hours of turbine usage in a 
reproducable manner. Many form of testing are available 



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today, and in general the better they match actual gas 
turbine conditions, the higher the cost, complexity, and 
required time. Pressurized burner rigs provide the closest 
laboratory simulation of actual turbine conditions {Ref. 6} 
by allowing control of gas pressures, velocities, composition, 
and temperature. Pressures up to 15 atm. and velocities up 
to mach 1.0 have been utilized to minimize the time required 
for corrosive attack. 

A less costly test method is the simple burner rig. 
This test apparatus consists essentially of a burner for the 
fuel, a combustion chamber, and a test chamber for the 
samples. Contaminants (salts, SOp, etc.) may be injected 
into the test chamber, mixed with the air supply, or mixed 
with the fuel supply (prior to combustion). Abnormally 
high levels of contaminants may be employed to obtain 
measurable attack within a few hundred hours, but for more 
consistent results, burner rig exposures of up to 5000 hours 
(approximately 7 months) have been recommended {Ref. 6}. 

A third type of Hot Corrosion testing involves the 
use of a laboratory furnace. In this type of test samples 
are placed in the furnace at the desired temperature and 
exposed to a flowing gas mixture of air and SO-. In addition, 
prior to this, the samples are sprayed with a salt solution 
ensuring a given level of salt film on the sample surface. 
In this way, the initiation phase of LTHC (the presence of 
a molten salt film) is essentially eliminated and a greatly 



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accelerated test is obtained. This is the type of test used 
in the Naval Postgraduate School Hot Corrosion Program. 
A simplified schematic of the tube furnace used at NPS is 
shown in Figure B.7. The total time required to produce 
typical Type 2 morphology is only 60 hours, and has given 
results comparable to those obtained using the more expensive 
and time consuming burner rig tests {Ref. 13}. 

A possible shortcoming of such an accelerated test 
method is that the relatively short time involved at high 
temperatures (60 hours) allows for very limited inter- 
diffusion between the substrate and coating, whereas in a 
typical gas turbine blade lifetime of 5000 hours, there 
is ample time for diffusion. The possible inter-diffusion of 
elements that may take place is illustrated in Figure 3.8. 
A process devised to more closely simulate the actual life 
of a turbine blade involves what is called pre-exposure . 
In pre-exposure, samples are exposed to a time/temperature 
environment that has been predicted to allow for the diffusion 
that would take occur in i0% of a turbine blade lifetime 
{Ref. 14.}. To minimize oxidation of the coating during 
pre-exposure, samples are vacuum sealed in quartz tubes. 
Further details of the pre-exposure process are described 
under PROCEDURES. Following pre-exposure, the samples 
are furnace tested as described above. 



20 



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2 . Previous Research 

The Hot Corrosion research program at the Naval 
Postgraduate School has been ongoing since 1979 and has 
focused primarily on substrate effects on Type 2 Hot 
Corrosion resistance. The following highlights the results 
of this program to date. Hafnium has been reported to be 
beneficial to LTHC resistance, but only up to some optimal 
(0.4. to 2.056) concentration, {Ref. 15}. Newberry {Ref. 16} 
conducted testing of uncoated superalloys, which included 
a study of the effect of Hafnium on the LTHC resistance of 
IN738. Pre-exposure was first used in 1981 and resulted 
in evidence of a detrimental effect on LTHC of inter- 
diffusion between the substrate and coating for some systems 
{Ref. 14-}. Jurey {Ref. 17} carried out a more extensive 
investigation of pre-exposure and reported on the overall 
degrading effects of pre-exposure on LTHC resistance. He 
also reported that a plantinum underlayer could be beneficial 
to LTHC resistance (the effect was sensitive to thickness), 
and observed that the maximum penetration measured on corroded 
test samples was very sensitive to pre-existing flaws, 
leaders, etc. In 1981, McGowen designed, tested, and 
validated the parameters used to perform Type 1 (high 
temperature) Hot Corrosion Testing. {Ref. 18} 



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C. OBJECTIVES 

A variable that has been difficult to control in the 
Hot Corrosion Program at the Naval Postgraduate School has 
been the SOp content of the furnace atmosphere. The low 
flow rate required (10 ml/min) is difficult to maintain by- 
most needle valves and is very sensitive to line pressure. 
The SOp flow rate has been observed to drop in this study 
from 15 ml/min to 5 ml/min in 5 hours. Type 2 Hot Corrosion 
resistance has been shown to vary significantly with change 
in SOp flow rates at NPS {Ref. 13 and 16}. For this 
reason one or two control pins were inserted in each 
furnace run of this study to determine if variations in 
SOp flow rate (and/or possibly other parameters) were 
affecting the corrosive conditions of the test. In addition, 
L samples in Run MS1 were re-tested in Run MS7 as a basis 
for determining what kind of a modifying factor, if any, 
should be applied to the results of a given run to allow 
comparison of the results from different runs. Finally, 
an electronic flow controller has been installed, enabling 
accurate and continuous monitoring of the SOp flow rate. 

The objectives of this study were to determine the 
effects of varying the amounts of titanium and hafnium 
present in the alloy substrate and the presence or absence 
of both a thick and thin platinum underlayer on the Type 
2 Hot Corrosion behavior of BC-21 coated Rene' 80. Hot 
Corrosion testing was performed both with and without 
pre-exposure . 

22 



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II. PROCEDURE 

Test specimens consisted of nominally 0.6 cm. diameter 
pins of modified Rene' 80 superalloy. The modification 
consisted of varying the titanium and hafnium content of 
the alloy (Rene' 80 normally contains 5.0$ Ti , and 0.0$ Hf ) , 
see Table IV. One pin of each composition was coated with 
the CoCrAly coating BC-21 using the SB-PVD method. One pin 
of each composition received a platinum flash prior to 
application of a BC-21 coating, and one pin of each compo- 
sition received a 5-6 urn platinum undercoating (this thick- 
ness has been shown to be optimal in other coating/substrate 
systems) prior to application of a BC-21 coating. The 
platinum was applied by electrodeposition . 

The procedures described below and listed in Table II 
were developed and validated by Busch {Ref. 13). 

Each test piece, a cylindrical pin nominally 2.0 cm in 
length by 0.6 cm in diameter, was visually inspected for 
defects and its length and diameter accurately measured. 
The specimen was then inserted in an oven at 1 50 C for 20 
minutes, after which it was cooled, weighed on an analytical 
balance, and replaced in the oven for an additional 20 
minutes. After removel from the furnace the second time, and 
while still hot, the specimen was sprayed with a Na ? S0, 
4.0 mole % Mg-SO, solution and then returned to the furnace 



2 



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for 15 minutes. It was then removed, cooled, and weighed. 

This was repeated until a weight gain representing an average 

2 

of 1 mg salt per cm of surface area was obtained. 

After salting, the specimen was placed in a holder 
together with other similarly prepared specimens and inserted 
into a resistance type tube furnace as shown in Figure B.7. 
Once brough to the desired temperature (704. C), the specimens 
were exposed to a flowing gas mixture consisting of dry air 
(flow rate of 2000 ml/min) and 0.5$ S0 2 - This flow rate 
has been determined to give a gas velocity of 1 cm/sec 
{Ref. 17}. The specimens were exposed to this corrosive 
environment for 20 hours , after which they were removed from 
the furnace, cooled to room temperature, visually inspected, 
weighed, and resalted as described above. Three 20 hour 
cycles were used for a total exposure of 60 hours. 

Upon completion of the 60 hours, the specimens were 
examined visually and photographed (Figure B.6 shows a 
closeup of a typical sample) . The pins were then sectioned 
in three places and prepared for microscopic examination 
using standard metallographic procedures. 

Depth of corrosion measurements were taken every 
20 degrees around a face. The pit like nature of the 
attack allows the location of the original surface of the 
coating to be accurately determined. Recorded was the 
mean penetration, (typically the average of 54 readings), 
standard deviation of the mean, the maximum penetration, 
and the average coating thickness observed. The 

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pre-exposure treatment given to some specimens prior to LTHC 
testing consisted of sealing in evaculated quartz tubes. to 
minimize surface degradation and heat treating in an oven 
at 875°C for 200 hours. 

In addition to optical microscopy selected samples were 
examined using a Scanning Electron Microscope. Backscatter 
images, spectrums, and line traces were made in an attempt 
to determine the concentrations of the elements at different 
locations in the coatings and substrates, particularly in 
those specimens subjected to the pre-exposure treatment. 
Since line traces and backscatter images proved to be of 
little use in this regard (due to poor resolution of minor 
constituent changes), spectrums were taken at 5000X at 
three locations in the coating: at the coating/substrate 
interface, at the center of the coating, and just under the 
corrosion front near the top of the coating, as shown 
schematically in Figure B.9. From these spectrums moderate 
changes in elemental concentrations were distinguishable. 

As previously mentioned, at least one control pin was 
inserted in each run to allow direct comparison of test 
conditions between runs. An electronic flow controller 
was installed on 28 March 1983 and its calibration verified 
with a new rotometer. Run MS 6 was made using this to 
control the 30 ? flow rate. A final Run, MS7 was made 
containing several identical pins to check the reproducability 
of the results. 



25 



Du 
Mo 



III. DISCUSSION/RESULTS 

Table III lists the specimens tested which were re-tested 
in various runs. They are listed simply as "Type 1, Type 2, 
etc." to prevent any significance being placed on their 
compositions and were chosen originally for their availability 
Also included in Table III are the actual corrosion results 
(the mean and maximum penetration), and the adjusted mean 
penetration (to be explained). Figure B.10a and b is the 
graphical presentation of this data. It illustrates the 
variation in corrosive conditions between furnace runs. 
Similar variation of results between different runs has been 
seen in a review of previous studies {Ref. 16}, although 
the nature and cause of these variations were not known. 

Upon installation of the electronic flow controller for 
S0 ? flow rate control, it was observed that the original 
rotometer read approximately 100 ml/min when the new 
controlled read 10 ml/min. The original rotometer (believed 
to have been in use since 1979) was replaced with a new 
rotometer, which agreed closely with the electronic controller 
This is evidence that the rotometer previously in use had 
been giving incorrect flow rate readings (probably due to 
the corrosive nature of the S0~ gas) which probably began 
gradually such that no individual researcher was aware 
of a change in the actual flow rate. By 1983, when this 



26 



Di 



study was begun, a reading of 1 ml/rain from the original 
rotometer resulted in an actual S0~ flow rate of only 2 rnl/min, 
which was too small a flow to be easily controlled by the 
needle valve arrangement installed at that time. This led 
to the observation that the SO- flow rate tended to drop 
off with time (but seldom in a predictable manner). After 
the first run or two, it was possible to get a crude "feel" 
for how to best maintain a roughly constant 30 ? flow rate, and 
this is somewhat reflected by the improved reproducibility 
between runs MS4- and MS 5. It is recommended that the SOp 
flow rate be continually monitored by two different methods 
(the electronic flow controller and a new rotometer) , in 
order that any slight change in SOp flow is detected early 
and can be corrected. 

Figure B.10a also shows the high degree of consistency 
obtained between runs MS6 and MS 7 (the electronic flow 
controller was installed prior to run MS6). This supports 
the claim that variation in the SOp flow rate was primarily 
responsible for inconsistencies observed in past furnace 
runs and also illustrates the high degree of consistency 
obtained within a given run (note samples MS7-2 thru MS7-9). 

Due to the possibility of inconsistencies in corrosive 
conditions between runs the use of a modifying factor was 
thought to be useful. With run MS6 as a baseline and using 
control pin type 1 (since this was the most common control 
used) , a modifying factor was developed using the following 
formula : 

27 



Dl 

Mc 



MF = Mean (6)/Mean(x) 

where MF = Modifying Factor 

Mean(6) = Mean penetration of the control 

pin on run MS6 
Mean(x) = Mean Penetration of the control pin 
in run MSx 
The mean penetration data for each run was then multiplied 
by the modifying factor for the run. This "adjusted data" is 
shown graphically in Figure B.10b and shows fair correlation 
of data from different runs. 

Since the maximum penetration of a sample has been noted 
to be very sensitive to the presence of pre-existing flaws 
and leaders, and since it represents only a single area of 
localized attack, mean penetration (typically the average of 
over 50 measurements) is used for comparison and evaluation. 
This is consistent with methods used previously at NPS. 
For this reason only the mean penetration data has been 
adjusted, the maximum penetration has been included for 
completeness only. 

A. SUBSTRATE EFFECTS 

Table IV lists the samples that were tested as part of 
this study. As mentioned previously the concentration of 
Titanium and Hafnium were varied as was the presence of and 
thickness of a platinum underlayer. Each type of sample was 
tested with and without pre-exposure. The sample abbreviations 
listed were designed to indicate to the reader the approximate 



28 



Di 



titanium concentration, whether or not hafnium was present, 
if a platinum underlayer was present (and if so, a thin 
platinum flash or a thicker, 5-6um underlayer), and also if 
the sample was pre-exposed. Also included are the original 
serial numbers for possible use by future researchers 
reviewing this study. 

Table V lists the samples by their abbreviations, along 
with the mean and maximum penetration, as mentioned previously, 
the mean penetration data has been adjusted by the use of 
modifying factors, and this is the data used in evaluation 
of results. 

The mean penetration data in Table V are presented 
graphically in Figures 3.11, B.12, and B . 1 3 . Each figure 
contains 4- or 6 individual graphs, and presents the entire 
data of Table V, but each in a slightly different manner in 
order to highlight specific trends. All the figures plot the 
tabulated corrosion behavior as a function of titanium 
concentration for ease of correlation. In Figure B.11 there 
are 6 plots, each for a specific platinum underlayer and 
hafnium combination, with a comparison cf with/without 
pre-exposure evident on each individual plot. This set 
primarily illustrates the effect of titanium. Figure B.12 
contains 4- plots, each for a specific combination of hafnium 
and pre-exposure, and primarily illustrates the effect of 
platinum underlayers. Figure B.13 contains 6 plots, each for 
a specific combination of platinum underlayer and pre-exposure, 



29 



and best illustrates the hafnium effect. Due to the number 
of variables involved (which results in 36 different types 
of specimens), the attempt was made to highlight trends 
rather than the possible significance of a single specimen 
performing differently from another. Therefore a relative 
ranking of all specimens tested is not included or discussed. 
Finally, the data of most significance is for the specimens 
that were pre-exposed since this better reflects corrosion 
behavior well into the life of a turbine blade. The data 
for specimens not pre-exposed was used primarily as an aid 
in explaining why certain results are obtained (i.e., if a 
trend is diffusion related, then it would show an effect 
due to pre-exposure ) . Therefore unless otherwise stated, 
hereafter any trends or other observations noted are assumed 
to be made for pre-exposed samples. 

Figure 3.11a and b show a decrease in mean penetration 
as Ti content is increased. This trend is either not as 
distinct or not observed in the samples which were not 
pre-exposed. Observations previously made on the effect of 
titanium were usually the result of the testing of different 
substrates (i.e., IN738 and Rene' 80) from which a distinct 
effect of titanium was difficult to discover { Ref . 1 4) . 

It was noted here that Ti was beneficial to LTHC resistance 
only if the time/temperature history of the sample was 
sufficient to allow for diffusion. In particular, the mean 
penetration of the following samples from Figure 3.11a: 



30 



Di 

M< 



(2-0-0), (5-0-0), (2-0-0-5), (5-0-0-5) distinctly indicates 
the advantage of allowing diffusion to take place for both 
low (2. Of.) and high (5. Of,) Ti content. 

High magnification (5000X) x-ray backscatter spectrums 
of (5-0-0) and (5-0-0-5) were made. While the spectrums 
obtained are by no means quantitative, they can give an 
indication as to whether diffusion is occurring. The spectrums 
obtained from the center of the coating of these samples 
are shown in Figure B.14-. The only difference is the presence 
of a small peak due to Ni in (5-0-0-5), indicating slight 
diffusion of Ni into the coating. While titanium was not 
observed in either case, this does not preclude the possible 
diffusion of titanium. This is because spectrums taken well 
into the substrate indicate that 5% titanium is roughly the 
minimum concentration required to be distinguishable by SEM 
backscatter analysis. Additionally, Katz (Ref. 19) has 
reported on the presence of titanium at the surface of 3C-21 
coated Rene'80. Ni was not observed in the spectrums taken 
at the outer edge of the coating on either sample. A 
backscatter dot-map and line probe for chromium was made on 
(5-0-0-5). These both indicated an enhanced concentration 
of chromium at the coating/substrate interface. This was 
not observed on (5-H-0-S). Since only a small number of 
samples were subjected to examination with the Scanning 
Slectron Microscope, it is not known if this enhanced 
chromium concentration at the interface is present in other 



31 



samples, or if it results from the coating process, rather 
than as a result of high temperature exposure. At this time, 
the chromium concentration noted has not been observed 
in other samples. 

When platinum is present, Figures B . 1 1 e and f indicate 
titanium is detrimental to LTHC resistance. In particular, 
sample (2-0-PP-E) performed much better than (5-0-PP-E). 
Figure B.11e and f also dramatically show that without pre- 
exposure this trend is reversed. SEM spectrum analysis was 
performed on (5-0-PP) and (5-0-PP-E) and the spectrums from 
the center of the coatings are included in Figure B . 1 5 • 
The spectrum for (5-0-PP) shows no platinum and no nickel. 
On (5-0-PP-E), a definite platinum peak is observed, as well 
as nickel. The chromium and cobalt contents at the coating 
center of (5-0-PP-E) are possibly lower than those at the 
coating center of (5-0-PP). 

Figure B.12b and d indicate that platinum (if the 
titanium content is high) is detrimental to LTHC resistance. 
Comparison of data on Figures B.12a and b show this effect 
to be reversed when pre-exposure is not performed. However, 
Figure B.12b and d show slight benefit of adding platinum 
underlayers to samples with only 2% titanium. SEM back- 
scatter analysis was not performed on samples (2-0-PP) and 
(2-0-PP-E), but it is thought that platinum has diffused into 
the coating in sample (2-0-PP-E), based on the diffusion noted 
in (5-0-PP-E). Previous studies by Clark {Ref. 20} have 



32 



shown the beneficial effect of platinum underlayers on 
CoCrAly coatings. 

A possible explanation of these observations is as follows. 
Without a platinum underlayer, and given the opportunity for 
diffusion, titanium diffuses outward into the coating where 
it produces a beneficial effect. Therefore, the higher the 
titanium concentration in the substrate, the better the LTHC 
resistance. 

When platinum underlayers are applied and an 
opportunity for diffusion (i.e., during exposure) exists 
the platinum underlayer can improve LTHC resistance. However, 
the titanium in the substrate may react with the platinum. 
At the 2% titanium level, titanium cannot diffuse due to 
this interaction, but there is enough excess platinum 
(notably in the 5-6 um thick underlayer) to allow sufficient 
platinum to diffuse outward to be of benefit. 

At the 3.5% titanium level, the sample without a platinum 
underlayer exhibits better LTHC resistance due to the additional 
diffusion of titanium. The sample with platinum underlayers 
shows little change, since the titanium remains at the interface 
At the 5% titanium level, however, there is sufficient 
titanium for some titanium to diffuse through the platinum 
underlayer into the coating, where it combines with the 
platinum that has already diffused outward, forming compounds 
that prove to be detrimental to LTHC resistance. 



33 



This last suggestion is supported by two observations; 
1) sample (5-0-PP-E) performed significantly worse than 
(5-0-0-E), (2-0-PP-E) and (5-0-PP), which again indicates 
that platinum and titanium in combination have a detrimental 
effect, and 2) significant diffusion of platinum into the 
coating has been noted in (5-0-PP-E). 

Detailed microprobe analysis must be performed to determine 
if indeed titanium is diffusing into the coating in (5-0- 
PP-E), and (5-0-0-E), and if, and what, compounds are being 
formed. Additionally, since the lowest level of titanium for 
this study was 2%, testing of samples with 0% titanium , 
with and without a platinum underlayer, would more clearly 
indicate the role of platinum in LTHC resistance. 

The effects of hafnium on LTHC resistance proved to be 
difficult to discern. Figure B.13 was included to present 
these results. Comparing Figure B.13b, d, and f, hafnium 
seems to have a noticeable (but inconsistent) effect on the 
samples with 3 - 5 % Ti (with a platinum flash hafnium was 
beneficial, but with 5-6 um platinum hafnium was detrimental). 
At the 2% and 5% Ti levels, hafnium has no noticeable effect. 
On samples that were not pre-exposed (Figure B.13a, c, and e) 
the reverse seems to be true. Hafnium has a noticeable 
(but again inconsistent) effect at 2% and 5% Ti , and a 
negligible effect on the 3.5$ Ti samples. 

Optical examination was unable to distinguish any 
differences in the coating structure or corrosion attack due 



34 



to the presence of hafnium. Previous studies, mainly on 
the nickel based alloy IN738 which contains 3 . U% titanium, 
have shown an optimum level of hafnium to exist, which ranged 
from 0.4-$ to 2.0/6, depending on the study. The 1.2 to 1 . 5% 
hafnium level chosen for this investigation is within this 
range. Although evidence is lacking, it is possible that 
the normally beneficial effect of hafnium is being influenced 
by the interactions suggested above. 



35 



IV. CONCLUSIONS AND RECOMMENDATIONS 

Based on the data, tables, and figures discussed, the 
following conclusions can be made: 

1. The presence of titanium in the substrate (in the 
absence of a platinum underlayer) is beneficial to LTHC 
resistance, provided that the time/temperature environment 
allows for interdif fusion between the substrate and coating. 

2. The combined presence of titanium in the substrate and 
a platinum underlayer are detrimental to LTHC resistance, 
again if inter-diffusion between the substrate and coating 
occurs . 

3. Platinum underlayers are detrimental to LTHC resis- 
tance if the titanium content of the substrate is relatively 
high (5.0$), again if inter-diffusion occurs. 

i. Hafnium has a definite, but inconsistent effect on 
LTHC resistance when intermediate levels of titanium {3.5%) 
are present in the substrate; this effect again depends upon 
inter-dif fusion. 

5. Hot Corrosion testing conducted prior to the 
installation of the electronic SO- flow controller that 
required correlation of data collected by separate furnace 
runs should be reviewed to determine if direct comparison 
between runs was justified, or if modifying factors should 
be applied to the results. 



36 



6. For Hot Corrosion testing conducted subsequent to the 
installation of the electronic S0~ flow controller direct 
correlation of data obtained from different runs is justified. 

The above conclusions and the previous discussion lead 
to the following list of recommendations: 

1 . Conduct a detailed, quantitative microprobe analysis 
of selected specimens used in this study in an attempt to 
determine more precisely diffusional changes that are 
occurring in order to better explain the behavior and effects 
noted above. 

2. Test samples with 0.0% titanium, and also 10.0% 
titanium (with and without platinum underlayers) to obtain 
additional data to support or refute the suggested explanations 
of the effects observed in the study. 

3. Perform a similar study on 3C-21 coated IN738, an 
alternative substrate alloy. This would require a study of 
the effect of hafnium and platinum only. 

4.. Review previous NPS research for possible intra-run 
comparisons that may not be justified, and, if possible, 
re-evaluate the data obtained using modifying factors and, 
based on this adjusted data, re-examine the results previously 
obtained. 



37 



APPENDIX A 
TABLES 



CQ 



i 

U 
PQ 

C 

o 
oo 



C 
<D 

pi 
o 

C 

o 

•H 

•M 
•H 

CO 

O 

CU 

6 
o 



03 
O 
•H 

g 



03 

•H 

e 
o 

2 



O 

2 



o 



O 



o 

LD 






O 

to 



o 



to 

o 



o 



o 



to 






ol 

pa 



03 
PQ 


i 

i 


o 




oo 




^ 


i— i 


0) 


rj 


c 


i 


<D 


o 


o£ 


pa 



38 



TABLE II 



TEST PARAMETERS 



Pre-Exposure 



preliminary - vacume sealed in quartz tubes 
temperature - 875 C 
time - 200 hours 



Type 2 Hot Corrosion 

air, source - laboratory air 

flow rate - 2000 ml/min 
S0 2 , source - bottled gas 

flow rate - 10 ml/min CO. 5 % ) 
salting, type - Na 2 S0 4 40 mole* MgS0 4 

amount - 1.0 mg/cm 
temperature - 704 C 
thermal cycle - 20 hours 
number of cycles - 3 
total time - 60 hours 



39 



E-t 



co 



H 

< 
c_> 
rH 

►J 

(X. 
Q 

O 

CO 
f- 

►J 
D 

CO 

w 

PS 
Q 

< 
CJ3 

r-( 

H 
CO 



Bd 

CD 
<U 

+-> 

M 

< 



c 

o 

•H 
+-> 

U 
+-> 

<1> 

C 

<u 
a. 



o 



crl 

C 



c 
o 

•H 

co h E 

CO 4-i 3- 

a, 



u 

• C 
-t $_ .H 

G 

H i—( I 
H -H • 

i- I- U 
CO 

c 

P 
OS 



rt 

H 

a, ex, 
6 a 

ccj o 

|co u 

CJ3 



r-- r- r- 

• • • 

to to to 

cm cm cm 



cm in 

vO vO 



vO 

to 

CM 



a* 
• i 

CM 



Csl 



to 



NNOOHCOl/lHONH^O 



o cm 



CM LT. 



rf LO 



O vO 



vO 



t*« r-- cm c^ 

to cm to to 



O vO 



to cr> lo 
o to oo 



vO vO 
vO r»» 



vO O 



d lti a> vo 



Ol^NNCimvOHHOlvOM 



t^r^oto^-NLnHtoioi/) 
rHCMCMCMCMCMCMCMCMCMCM 



vO vO 

•3- in 



a> 



r^ vO 

CM 



vO vO 
OO rH 



•^- rH 



CM 



O CM 

rH CM 



tO CM ^ 

NHHHN^rr^vOh-OOOl 

I I I I I I I I I I I I 

rH«rt-L/"ivor^-r-~-r-~r^r^r~^r^r-~ 

cocococococococococococo 



CM 


to 


rj- 


LO 


r>» rH 


CM rH 


OO rH 


LT> rH 


t vO 


rH r^ 


th c-- 


rH r-. 


co co 
"2. S 


CO CO 

25 S 


CO CO 


CO CO 



CM CM 



to to 



rj" ^3- 



in lt> 



40 



TABLE IV 



LISTING OF SAMPLES TESTED 



SUBSTRATE STUDY 





Sample 




Composition 




Original Serial Nr. 


Abreviation 


(ITi, 


*Hf 


. Pt, 


Exp . ) 


(Run Nr. - Pin Nr . ) 


(2-0-0) 


2.0 








no 


MS1-9 


(2-0-p) 


2.0 





flash 


no 


MS5-13 


(2-0-PP) 


2.0 





5- 6pm 


no 


MS1-3 


(2-H-O) 


2.0 


1.5 





no 


MS1-8 


(2-H-p) 


2.0 


1.5 


flash 


no 


MS5-9 


(2-H-PP) 


2.0 


1.5 


5 -6pm 


no 


MS1-2 


(3.5-0-0) 


3.5 








no 


MS5-6 


(3.5-0-p) 


3.6 





flash 


no 


MS5-11 


(3.5-0-PP) 


3.6 





5-6pm 


no 


MS5-8 


(3.5-H-O) 


3.5 


1.5 





no 


MS5-7 


(3.5-H-p) 


3.5 


1.5 


flash 


no 


MS5-2 


(3.5-H-PP) 


3 . 5 


1.5 


5-6pm 


no 


MS5-1 


(5-0-0) 


5.0 








no 


MSI -6 


(5-0-p) 


5.0 





flash 


no 


MS5-5 


(5-0-PP) 


5.0 





5 -6pm 


no 


MS5-10 


(5-H-O) 


5.0 


1.2 





no 


MS1-5 


(5-H-p) 


5.0 


1.2 


flash 


no 


MS5-4 


C5-H-PP) 


5.0 


1.2 


5-6pm 


no 


MS5-3 


(2-0-0-E) 


2.0 








yes 


MS4-9 


(2-0-p-E) 


2.0 





flash 


yes 


MS6-13 


(2-0-PP-E) 


2.0 





5-6pm 


yes 


MS4-3 


(2-H-0-E) 


2.0 


1.5 





yes 


MS4-8 


(2-H-p-E) 


2.0 


1.5 


flash 


yes 


MS6-9 


(2-H-PP-E) 


2.0 


1.5 


5-6pm 


yes 


MS4-2 


(3.5-0-0-E) 


3.5 








yes 


MS6-6 


(3.5-0-p-E) 


3.6 





flash 


yes 


MS6-11 


(3.5-0-PP-E) 


3 . 6 





5-6pm 


yes 


MS6-8 


(3.5-H-O-E) 


3.5 


1.5 





yes 


MS6-7 


(3.5-H-p-E) 


3.5 


1.5 


flash 


yes 


MS6-2 


(3.5-H-PP-E) 


3.5 


1.5 


5 -6pm 


yes 


MS6-1 


(5-0-0-E) 


5.0 








yes 


MS4-6 


(5-0-p-E) 


5.0 





flash 


yes 


MS6-5 


(5-0-PP-E) 


5.0 





5-6pm 


yes 


MS6-10 


(5-H-O-E) 


5.0 


1.2 





yes 


MS4-5 


(5-H-p-E) 


5.0 


1.2 


flash 


yes 


MS6-4 


(5-H-PP-E) 


5.0 


1.2 


5 -6pm 


yes 


MS6-5 



41 



TABLE V 



CORROSION RESULTS - SUBSTRATE STUDY 





Sample Me 


an Penetration Max. Penetration 


Abreviation 


( H m) 


( H m) 


(2-0-0) 


37.6 


76.2 


(2-0-p) 


18.0 


106.7 


(2-0-PP) 


35.8 


86.4 


(2-H-0) 


24.4 


101.6 


(2-H-p) 


27.2 


167.6 


(2-H-PP) 


22.6 


76.2 


(3.5-0-0) 


24.4 


91.4 


(3.5-0-p) 


21.3 


101.6 


(3.5-0-PP) 


17.8 


193.0 


(3.5-H-0) 


21.3 


190.5 


(3.5-H-p) 


22.6 


132.1 


(3.5-H-PP) 


20.8 


101.6 


(5-0-0) 


26.4 


81.3 


(5-0-p) 


13.0 


88.9 


(5-0-PP) 


15.0 


66.0 


(5-H-O) 


31.2 


66.0 


(5-H-p) 


23.4 


381.0 


(5-H-PP) 


20.6 


81.3 


(2-0-0-E) 


15.7 


81.3 


(2-0-p-E) 


18.8 


89.9 


(2-0-pp-E) 


8.6 


81.3 


(2-H-O-E) 


17.8 


81.3 


(2-H-p-E) 


15.0 


152.4 


(2-H-PP-S) 


14.0 


279.4 


(3.5-0-0-E) 


11.7 


177.8 


(3.5-0-p-E) 


15.5 


76.2 


(3.5-0-pp-S) 


9.7 


177.8 


(3.5-H-0-E) 


9.1 


132.1 


(3.5-H-p-E) 


5.6 


127.0 


(3.5-H-PP-E) 


23.6 


116.8 


(5-0-0-E) 


6.4 


71.1 


(5-0-p-E) 


18.3 


101.6 


(5-0-PP-E) 


22.9 


111.8 


(5-H-O-E) 


7.6 


96.5 


(5-H-p-E) 


22.9 


127.0 


(5-H-pp_e) 


24.1 


127.0 



U2 



APPENDIX B 



FIGURES 



High Pressure Turbine 



Combustor 



Compressor 



Power Turbine 




Power Shaft 



Figure B.1 Relative Temperature and Pressure Profile 

of a Marine Gas Turbine Engine 



43 



Substrate to 
be coated 




Water Cooled 
Crucible 



Magnet 



Electron Beam 
Source 



Figure B.2 Simplified Drawing of the Electron Beam 
Physical Vapor Deposition (SB-PVD) Process 



U 




v<^.vri^ 



Figure B.3 Typical CoCrAiY (3C-21) Coating on Rene 1 80 



45 



Na 2 S0 4 -CoS0 4 melt 


lso 3 |o 2 


Al +++ 


'sQ/~~ 


Al depleted alloy 




CoCrAlY 





(a) Onset of Type 2 Attack 




(b) Corrosion Front Continues to Disolve Coating 



log P 



0, 



MO 



M 




(c) Stability Diagram 



Reaction 
Path 



log P 



SO. 



Figure 3.4 



Type 2 (Low Temperature) Hot Corrosi.on, 
Simplified Schematic 

46 








Figure B. 5 



Typical Type 2 Hot Corrosion in CoCrAlY (BC-21) 

Coating 



kl 




Figure B.6 



Typical Type 2 Hot Corrosion on BC-21 Coating 
Macrophoto (enlarged 7.5 x) 



48 



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o 



CM 

O 
co 




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O 

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o 



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CD 

M 

•H 



49 




Substrate Coating 



Protective 
Oxide 



Typical X elements 

Ni 

W 

Mo 

Ti 

Hf 

Pt 



Typical Y elements 

Al 

Cr 

Co 

Y 

Pt 



Figure B.8 - 



Schematic Illustration of the Substrate/Coating 
Diffusion Process 



50 




r\< Top of 

Coating 




O 



.Center of 
Coating 




Coating/Substrate 
Interface 



Figure B.9 Schematic Illustration of the Method of Using 
High Magnification Spectrochemical Analysis for 

Diffusion Study 



51 




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on Behavior of BC 
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61 




a) 5% Ti 




b) 5% Ti, with Pre-Exposure 



Figure B . 1 4. Chemical Spectrums of Center of BC-21 Coating 
on Corroded Rene' 30 (5% Ti Modification) 



62 




a) 5% Ti, 5 - 6 um Pt. 



' 






r.TMwgar T -WwirgiMil 


Co 


■wj..^,..- miTfVirri^ pa». i ,| ri«iKBiinrtwfi-w.^. s 


1 
. A\ A 


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ft 




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1 




Ni 


J 




Li>i ^.-^ , ,iao. 


M i 


M 




!*■ 


■ 


mmgi 


imIm 


Mifln 


JL*. -A. 









b) 5$ Ti, 5-6 um Pt, with P 



re-exposure 



Figure B.15 Chemical Spectrums of BC-21 Coating on Corroded 
Rene^ 80 (5% Ti Modification) with Platinum 

Underlayer 

63 



LIST OF REFERENCES 



1. Hawkins, P.F., "LM 2500 Operating Experience on GTS ' 
CALLAGHAN, " Proceedings of the 4th Conference on Gas 
Turbine Materials in a Marine Environment , pp. 39-69 , 
Annapolis, MD, June 1979. 

2. Boyer, H.E., ed., Metals Handbook , V. 10, American 
Society for Metals, 1975. 

3. Brick, R.M., Gordon, R.B., and Pence, A.W., Structure 
and Properties of Engineering Materials , Dp. 387-390, 
McGraw-Hill, 1977. 

4. Jaffee, R.I., National Materials Advisory Board, Report 
NMAB-260, Hot Corrosion in Gas Turbines , May 1970. 

5. Fontana, M.G., and Green, N.D., Corrosion Engineering 
pp. 361-367, McGraw-Hill, 1978. 

6. Sims, C.T. and Hagel, W.C., eds., The Superalloys , 
Wiley, 1972. 

7. King, R.N., An Investigation of the Substrate/Platinum 
Effect in Low Temperature Hot Corrosion of Marine Gas 
Turbine Materials , Master's Thesis, Naval Postgraduate 
School , June 1 q 81 . 

8. Aprigliano, L.F., David W. Taylor Naval Ship Research 

and Development Center Report TM 28-78/218, Low Temperature 
(1300°F) Burner Rig Test of MCrAlY Composition Variations , 
September 26, 1978V 

9. Fairbanks, J., "Ceramic Coating Development, A Technical 
Management Perspective," Proceedings of the 4-th Conference 
on Gas Turbine Materials in a Marine Environment , 

pp. 749-764, Annapolis, MD , June 1979. 

10. Jones, R.L., Naval Research Laboratory Memorandum 
Report 4072, A Summary and Review of NAVSEA Funded Low 
Power Hot Corrosion Studies , Washington, D.C., 
September 24, 1979. 

11. Jones, R.L., Naval Research Laboratory Memorandum 
Report 5070, Hot Corrosion in Gas Turbines , Washington 
D.C. , April 27, 1983. 

12. Luthra, K.L., and Shores, D.A., "Morphology of Na ? S0, 
Induced Hot Corrosion at 600-750°C" Proceedings 4 
of the 4th Conference on Gas Turbine Materials in a 
larine Environment , pp. 525-542, Annapolis, MD, June 1979. 



64 



13. Busch, D.E., The Platinum Sffect in the Reduction of 
Low Temperature " Hot Corrosion on Marine Gas Turbine 
Materials , Master's Thesis, Naval Postgraduate School, 
December 1980. 

1 £. Collins, J.G., The Substrate Sffect in Low Temperature 
Hot Corrosion of Marine Gas Turbine Coating Materials , 
Master's Thesis, Naval Postgraduate School, December 
1981 . 

15. Exell, J.R., The Substrate Sffect of Active Element 
Hafnium in Aluminide Coatings, Master's Thesis, Naval 
Postgraduate School, June 1981. 

16. Newberry, G.D., Studies of Low Temperature Hot Corrosion 

of Uncoated Superalloys, Master's Thesis, Naval Postgraduate 
School, September 1981. 

17. Jurey, S. N., Substrate Effects on Hot Corrosion 
Resistance of Nickel Base Superalloys , Master's 
Thesis, Naval Postgraduate School, June 1982. 

18. McGowen, T.L., Type 1 Hot Corrosion Furnace Testing 
and Evaluation , Master's Thesis, Naval Postgraduate 
School, Monterey, California, October 1982. 

19. Katz, G.B., and Boone, D.H., Lawrence Berkely Labora- 
tory, University of California, Berkely, California 
Private Communication. 

20. Clark, R.L., "Low and High Temperature (704°C and 899°C) 
Burner Rig Evaluations of Advanced MCrAly Coating Systems," 
Proceedings of the 4-th Conference on Gas Turbine 

Materials in a Marine Environment , pp. 139-220, 
Annapolis, MD, June 1979. 



65 



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No. Copies 



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2. Library, Code 0U2 2 
Naval Postgraduate School 

Monterey, California 93940 

3. Department Chairman, Code 69 1 
Department of Mechanical Engineering 

Naval Postgraduate School 
Monterey, California 9394-0 

4-. Adjunct Professor D. S. Peacock, Code 69 2 

Department of Mechanical Engineering 
Naval Postgraduate School 
Monterey, California 9394-0 

5. Adjunct Professor D. H. Boone, Code 69bi 6 
Department of Mechanical Engineering 

Naval Postgraduate School 
Monterey, California 9394-0 

6. Mr. Louis F. Aprigliano, Code 2812 2 
David W. Taylor Naval Research and 

Development Center 
Annapolis, Maryland 214-02 

7. LT Michael J. Shimko 2 
64-09 Fair Oaks Avenue 

Baltimore, Maryland 21218 



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Shimko 

An investigation 

substrate effects ( 

type two hot corrosion 

of marine gas turbine 

materials. 



Thesis 

S4735 

c.l 



202121* 



Shimko 

An investigation of 

substrate effects on 
type two bot corrosion 
of marine gas turbine 
materials. 



An investigation of substrate effects on 




3 2768 001 95322 7 

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